In current integrated gasification combined-cycle (“IGCC”) power generation systems, an air separation unit is used to supply O2 to a gasifier, which then generates partially combusted gases for use as fuel in a gas turbine. Compressed air is generally supplied to the air separation unit from both a main air compressor and/or through extraction from the discharge of the gas turbine compressor. Currently, the amount of compressed air extracted from the turbine compressor discharge is approximately a fixed percentage of compressor flow and is based only on the needs of the external demand of the air separation unit.
In such systems, the goal for the operation of the gas turbine is to satisfy desired load levels while maximizing efficiency. This includes allowing the gas turbine unit to operate at a desired maximum level load across changing ambient conditions without exceeding the maximum load level, while also respecting operational boundaries of the turbine. Operational boundaries, for example, include maximum allowable temperatures within the turbine or combustor components. Exceeding these temperatures may cause damage to turbine components or cause increased emissions levels. Another operational boundary includes a maximum compressor pressure ratio. Exceeding this limitation may cause the unit to surge, which may cause extensive damage to the turbine. Further, the turbine may have a maximum mach number, which indicates the maximum flow rate of the combusted gases as the gases exit the turbine. Exceeding this maximum flow rate may damage turbine components.
Accordingly, controlling the operation of the gas turbine to improve efficiency while satisfying operational limitations or requirements is a significant goal within the industry. Several known methods are used by turbine operators to control or limit the load of the turbine while attempting to satisfy this objective. These known methods include manipulating inlet bleed heat, the inlet guide vanes of the compressor, and/or turbine fuel supply.
Inlet bleed heat allows a turbine operator to bleed off the discharge air of the turbine compressor and recirculate the bleed air back to the compressor inlet. Because some of the compressor flow is recycled to the inlet, this method reduces the amount of flow through the compressor that expands through the turbine, which reduces the output of the turbine. This method of gas turbine load control may also raise the inlet temperature of the compressor inlet air by mixing the colder ambient air with the bleed portion of the hot compressor discharge air. This rise in temperature reduces the air density and, thus, the mass flow to the gas turbine. Although this approach may be used to allow the gas turbine unit to operate at a maximum level loaded across changing ambient conditions (while respecting operational boundaries), it comes with a cost, as it reduces the thermal efficiency of the gas turbine.
Closure of the inlet guide vanes, which control the flow of air to the turbine compressor, is another common method of decreasing the mass flow across the gas turbine, which, in turn, may be used to control or limit turbine load. Closing the inlet guide vanes may restrict the passage of air to the compressor and, thus, decreases the amount of air entering the compressor. This approach also may be used to allow the gas turbine unit to operate at a maximum level load across changing ambient conditions (while respecting operational boundaries), but it also reduces the thermal efficiency of the gas turbine by operating the compressor away from its optimum design point.
Finally, the turbine load may be controlled or limited by decreasing the flow of fuel to the combustor. This will decrease the combustion temperature of the turbine and the output of the gas turbine engine. In the case of falling ambient temperatures, such a measure may allow the turbine to maintain a maximum level load. However, as is known in the art, the reduction in combustion temperature decreases the efficiency of the gas turbine engine.
These known control methods thus adversely affect the efficiency of the gas turbine engine. Further, none of these control methods take advantage of the specific components that are part of an IGCC power generation system to allow the system to operate more efficiently. Thus, there is a need for a more efficient method for controlling the load of the gas turbine used in an IGCC power generation system.